1. Field of the Invention
The present invention relates to a laser light generating apparatus for functioning as a light source in an optical apparatus such as an ultraviolet microscope and an inspection apparatus for various types of electronic materials and electronic components and so forth. More particularly, the present invention relates to a laser light generating apparatus for generating ultraviolet light with a short wavelength of about 200 nm or less.
2. Description of the Related Art
Laser light has coherence, since the laser light is superior in monochromaticity and directivity because of its single wavelength and phase. Moreover, the laser light can be very finely focused, and thus a tiny area can be irradiated with the laser light. Furthermore, the laser light generally has a higher frequency than a radiowave has, and thus the laser light can have a larger information capacity. Because of the above-mentioned characteristics, the laser light is applied to various fields such as the field of information and communication processing, the field of microfabrication, the field of measurement and the field of medical care.
The performance of an apparatus using the above-mentioned laser light as a light source is generally determined by the wavelength of the laser light and the output stability thereof. Therefore, a laser of a short wavelength has been heretofore developed, but, above all, laser light with wavelengths of about 180 nm to 204 nm has not been obtained yet in the form of continuous light capable of fully satisfying the conditions for practical use, and thus, pulse light alone has been used. It is therefore difficult to use the laser light with wavelengths of about 180 nm to 204 nm in the technical field such as the mastering of an optical disk requiring continuous light or quasi-continuous light such as mode-locked light. When pulse light is used in a semiconductor exposure system or a microstructure inspection apparatus, an optical system, an object to be exposed or inspected or the like may therefore suffer damage due to high peak power of the pulse light.
Continuous light with a wavelength of about 204 nm or more can be obtained with relative ease in the following manner: for example, light with a wavelength of 408 nm or more is entered into a BBO (beta barium borate: xcex2-BaB2O4) crystal cut at a phase matching angle so that second harmonic generation (SHG) takes place. However, this method has not so far achieved the generation of continuous light with a wavelength of about 204 nm or less.
On the other hand, reported is a method in which ultraviolet rays with shorter wavelength (about 200 nm or less) undergo continuous wave oscillation by means of the sum frequency of near infrared light and ultraviolet light (about 200 nm to 400 nm). For example, Watanabe et al. has proposed an apparatus in which light with a third wavelength of 194 nm is generated through the entry of both SHG of an argon ion laser and titanium sapphire laser light into a nonlinear optical crystal (M. Watanabe et al., Optics Communications, vol. 97, pp. 225-227 (1993)). In Japanese Patent Application Publication No. Hei 10-341054, proposed is an apparatus in which light with a third wavelength of 193 nm is generated through the sum-frequency mixing of the fourth-order harmonic of a YAG laser and titanium sapphire laser light. These techniques are adapted to improve conversion efficiency by resonating both the two above-mentioned input waves, and a resonator has a structure in which part of an optical path in the resonator is shared by the two input waves.
However, it is very difficult that optical components arranged on the shared optical path are coated so as to satisfy the conditions for low loss of both the respective wavelengths of the input waves. Therefore, the above-mentioned structure has a problem: that is, the finesse of the resonator decreases and thus it is difficult to efficiently perform wavelength conversion. For example, the experimental results obtained by Watanabe et al. are that the factor of multiplication of ultraviolet light by the resonator is limited to about 5 times.
In the latter example, the generated ultraviolet light with the third wavelength is separated from two incoming light beams by a wavelength separating mirror. This mirror also requires high reflectance for two input waves and high transmittance for output waves, but, as in the former case, the mirror is difficult to realize, and thus it is difficult to perform efficient wavelength conversion. In this case, when the intensity of output light is increased, two high-intensity ultraviolet rays overlap each other on a coaxial optical path, and this may result in the destruction of the apparatus. To meet a demand to secure ultraviolet light durability of an optical system in the above-mentioned case of high output, this demand limits a coating material and can thus lead to a decrease in conversion efficiency.
Furthermore, a titanium sapphire laser for use in these techniques has a very wide range of tunable frequencies, and thus, in order to stabilize an absolute wavelength of the laser for the long term, it is necessary to lock the absolute wavelength on a reference resonator or refer to an absorption line of a gas, so that the apparatus becomes complicated.
The invention is designed to overcome the foregoing problems. It is an object of the invention to provide a laser light generating apparatus which is capable of oscillating continuous light with a range of wavelengths of about 200 nm or less with stability at high conversion efficiency and is capable of size reduction, and an optical apparatus using the same.
In a laser light generating apparatus of the invention, laser light with a first wavelength and laser light with a second wavelength, which are spatially separated from each other, enter into a nonlinear optical device through an incident surface, and the laser light with the first wavelength, the laser light with the second wavelength and laser light with a third wavelength, which are spatially separated from one another, exit from the nonlinear optical device through an exit surface and have different optical paths. When at least either incident angles or exit angles of the laser light with the first wavelength and the laser light with the second wavelength to the nonlinear optical device are set according to the wavelengths thereof, at least either the incident angles or the exit angles are effective. More particularly, when the incident angle or the exit angle of at least one of the laser light with the first wavelength and the laser light with the second wavelength to the nonlinear optical device is equal to a Brewster angle, the incident angle or the exit angle is effective.
In the laser light generating apparatus of the invention, the laser light with the first wavelength and the laser light with the second wavelength enter into the nonlinear optical device at different incident angles, so that entry conditions are determined according to the wavelengths thereof, and therefore losses in first and second resonators are reduced. When the laser light with the first wavelength and the laser light with the second wavelength have the same refraction angle to the nonlinear optical device, their optical axes become parallel to each other in the nonlinear optical device, or, when the laser light with the first wavelength and the laser light with the second wavelength have different refraction angles to the nonlinear optical device, their optical axes do not match in the nonlinear optical device. Therefore, the laser light with the first wavelength and the laser light with the second wavelength do not share an optical path, except when their beams overlap for sum-frequency mixing.
An optical apparatus of the invention comprises the laser light generating apparatus of the invention and is applied to a microscope, various types of analysis and inspection apparatuses, a disk mastering apparatus and so forth.
Other and further objects, features and advantages of the invention will appear more fully from the following description.